To eliminate this danger of uneven exposure, a large flashbulb must be used. Such a large flashbulb, however, is not very convenient, because it requires a large-sized flash device and such a special large-sized flash device is not easily available in the market.
For the purpose of solving this problem an elongated flashlight duration may be obtained by flashing a plurality of flashbulbs one by one at time intervals. By this method, however, a flashlight having a long duration and a flat characteristic cannot be obtained only by flashing a plurality of flashbulbs one by one at time intervals, because each flashbulb has such a flashlight characteristic that it rapidly reaches the peak level in the early period of flashing and reduces its brightness gradually in the end of flashing.
FIG. 1 represents a flashlight characteristic curve a of a single small flashbulb in comparison to an ideal flashlight characteristic curve b required by a large-sized camera.
FIG. 2 is a graph for explaining what type of composite flashlight characteristic can be obtained by flashing two separate small flashbulbs having equal flashlight characteristic curves as shown at a and a'. As is mentioned previously, each flashbulb has a large light increasing rate in the early period of flashing and small light reducing rate in the end of flashing. Therefore, the composite flashlight obtained by composing these flashlights will not have a flat characteristic but include a peak c at each overlapping position. Such a peak of flashlight will cause an unevenness of exposure.